The 500th alien world appears to have been discovered. The milestone was reached Nov. 19, according to the Extrasolar Planets Encyclopedia, a database compiled by astrobiologist Jean Schneider of the Paris-Meudon Observatory.
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Last month, astronomers announced the discovery of the first potentially habitable extrasolar planet. But this week at an International Astronomical Union meeting, doubts were raised about the existence of this exciting new planet said to be orbiting the star Gliese 581.
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Space.com: Several extrasolar planets have been discovered to be orbiting backwards — that is, they revolve in the opposite direction that their host star rotates — challenging accepted ideas of how planets form, according to the astronomers who made the discovery.
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